1. Field of the Invention
The invention relates to fuel cells, and in particular to monopolar fuel stacks.
2. Description of the Prior Art
Although there are five primary types of fuel cells, one of the most common types is the polymer electrolyte membrane (PEM) fuel cell. A PEM fuel cell consists of several membrane electrode assemblies (MEAs) within gas diffusion layers and bipolar plates. The purpose of a fuel cell is to produce an electrical current. The chemical reactions that produce this current cause the fuel cell to function. In general terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons. The hydrogen atoms are now “ionized” and carry a positive electrical charge. In some cell types oxygen enters the fuel cell at the cathode and combines with electrons returning from the electrical circuit and hydrogen ions traveling though the electrolyte from the anode. In other cell types the oxygen picks up electrons and then travels through the electrolyte to the anode where it combines with hydrogen ions. Regardless of whether oxygen and hydrogen combine at the anode or cathode, together they form water, which drains from the cell. As long as a fuel cell is supplied with hydrogen and oxygen it will generate electricity.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
Currently stacks are fabricated with bipolar stacks where the majority of the mass is associated with a bipolar plates that serve to electrically connect cells and distribute the fuel and oxidant. This type of design a “bipolar plate” serves as a repeating element that serves to interconnect the cells and distribute the reactants. Such a bipolar stack is held together under pressure by “end plates” to ensure good contact and sealing. A typical stack would have two or more bipolar plates. The bipolar plates are usually fabricated from graphite composites while the end plates are made from titanium, stainless steel or aluminum. Several tie rods usually run across the stack to hold the plates together. The bipolar stack has the advantage of providing a very low internal resistance which is crucial for minimizing the losses for large currents that may flow through these stacks, and which is especially necessary for stacks which output more than a few tens of watts.
However, when the power output is only a few watts, the very low internal resistance offered by the bipolar stack design is not absolutely necessary. Such bipolar plates and end plates are usually machined or molded with flow field from graphite composite and the typical cost $50-$100/sq. foot, and become a major part of the costs of the stack. Most importantly, the biplates, end plates and tie rods constitute about 80% of the weight of a typical stack thus lowering the power density of the stack. Also, once such as stack is assembled, trouble shooting will require the entire stack to be dismantled if any of the cells in the center of the stack has to be accessed.
Therefore, a new design that is substantially less expensive to fabricate, lighter, does not require extensive pressure to ensure sealing, that eliminates biplates and endplates totally, and is easy to manufacture and troubleshoot, is highly desirable for commercialization of fuel cells.